All about it
The MM315 Balanced Modulator is a ring modulator with surprising clarity and precision that belies its "vintage" character, enabling it to do double duty as a high-quality VCA with handy utilities (patchbay and attenuators) as a bonus.
For the uninitiated, ring modulation differs from a standard VCA by allowing negative multiplication of signals as well as positive. It was used in many classic synths and electronic music studios to allow the generation of clangorous metallic tones (not to mention Daleks and Cybermen!) from two simple input waveforms of differing frequencies. For this reason it could be thought of as the "East Coast Wavefolder".
The MC1496 modulation chip from the late 1960s has passed into legend and is still popular today, however the original AR315 was about the only use of its more sophisticated, much better-performing, analogue computing grade sibling the MC1595L, which is now out of production. We have sourced new-old-stock examples of this chip for the MM315.
The result is an excellent ring modulator which has very low distortion and does not "lose tone" as amplification is increased, unlike MC1496 based circuits. Indeed, it can easily serve as a good quality conventional output VCA if paired with an envelope generator that outputs zero volts when idle.
The two outputs (MOD1 and MOD2) are identical and are independently connected to the output signal via 1k protection resistors. There is also a bonus 4-way patchbay and two 100k passive linear attenuators.
We have added a switch that can add in an AC COUPLING capacitor to both X and Y inputs for when both inputs are audio signals and you want to minimise carrier (or modulator) feedthrough.
Both inputs have very good performance but best results will be obtained by putting constant-level audio sources ("carriers") into the X input and variable level sources such as speech or envelope generators ("modulators") into the Y input, as the X input has almost no signal feedthrough or "bleed" when the Y input is not active.
For conventional VCA duties we have had good results when driven by a Mutable Peaks, a CGS/Serge Quadslope and a Doepfer A-140 ADSR. The MM312 EG may give a slight bleed due to its slightly negative null point.
The unit has been factory calibrated to give unity gain on the X input when +5V is present on the Y input. Please turn the Y input attenuator to about 6 when using +8V envelopes (most Euro envelopes), and 5 when using +10V envelopes (Envelator, or MOTM/Dotcom). Likewise, please turn the input attenuator to 5 when inputting an audio source which may exceed +5V, such as the output of the MM314 or the square wave of the MM317.
It's probably worth noting that this is the only member of the Capricorn Series where a change was made to correct a noteworthy design boo-boo in the original. When testing the unit on 12V supplies a distortion problem emerged, as well as an observation that varying attenuators or disconnection of jacks could cause signal bleed from the other channel. We went back to the MC1595 datasheet and found that this design was a pretty straight adoption of a reference design on the datasheet, which enabled us to recalculate a resistor value to work with 12V supplies. Unfortunately the AR315 designers had chosen to replace the fixed resistive input divider of the original with an input attenuator, which not only meant that signals could be put through the system at double the optimum signal level but that the unit would have the bleed problems mentioned above.
We have fixed this with the best of both worlds - the original fixed divider has been restored to optimise performance, and the input potentiometers are now buffered through a modern audiophile-grade opamp to prevent signal bleed issues.
+5V: Not required
Eurorack standard, 10HP width
38mm depth below mounting rails with connected power cable
User guide v1.01 (September 2016) [Print double sided and flip on short edge]
Calibration guide v0.1 (In progress)
In production; on sale
September 2016: Serials 600-000 to 600-011